Fantastic. I say this every time I read a Reimu and Marisa doujinshi, but I just can't get enough.
The sort of rushed, hurried mood through the first half of the book was nice. You get these quick flashes at the beginning of what happened, and then Marisa rushing to meet up with Reimu. I enjoyed that. I think you're meant to think, over time, about what actually happened. By the time she arrives and they apologize to each other, you kind of piece it all together then. (Marisa spurned Reimu's advances, which made Reimu sad, but Marisa was just too surprised at the time to process it, hence why she rushes so fast to meet Reimu after).
Really cute story, really cute resolution. Ending with them holding hands and falling alseep, just perfect.